error: at least one trait must be specified
  --> $DIR/issue-78720.rs:1:16
   |
LL | fn server() -> impl {
   |                ^^^^

error[E0412]: cannot find type `F` in this scope
  --> $DIR/issue-78720.rs:13:12
   |
LL |     _func: F,
   |            ^
   |
  ::: $SRC_DIR/core/src/ops/function.rs:LL:COL
   |
LL | pub trait Fn<Args>: FnMut<Args> {
   | ------------------------------- similarly named trait `Fn` defined here
   |
help: a trait with a similar name exists
   |
LL |     _func: Fn,
   |            ~~
help: you might be missing a type parameter
   |
LL | struct Map2<Segment2, F> {
   |                     +++

error[E0308]: mismatched types
  --> $DIR/issue-78720.rs:7:39
   |
LL |     fn map2<F>(self, f: F) -> Map2<F> {}
   |                                       ^^ expected struct `Map2`, found `()`
   |
   = note: expected struct `Map2<F>`
           found unit type `()`

error[E0277]: the size for values of type `Self` cannot be known at compilation time
  --> $DIR/issue-78720.rs:7:16
   |
LL |     fn map2<F>(self, f: F) -> Map2<F> {}
   |                ^^^^ doesn't have a size known at compile-time
   |
   = help: unsized fn params are gated as an unstable feature
help: consider further restricting `Self`
   |
LL |     fn map2<F>(self, f: F) -> Map2<F> where Self: Sized {}
   |                                       +++++++++++++++++
help: function arguments must have a statically known size, borrowed types always have a known size
   |
LL |     fn map2<F>(&self, f: F) -> Map2<F> {}
   |                +

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0277, E0308, E0412.
For more information about an error, try `rustc --explain E0277`.